Bistable circuit



BISTLE crncUrr James L. Burrows, Natick, Mass, assignor to Laboratory for Electronics, Inc., Boston, Mass, a corporation of Delaware Application Qctoher 28, 1953, Serial No. 338,89tt

7 Claims. (Cl. 25027) The present invention relates in general to electron tube trigger circuits and more particularly concerns a novel bistable circuit utilizing a single pentode and offering an unusual degree of reliability together with maximum economy of associated component parts.

The available designs and applications of bistable electron tube circuits, or flip-flops, are too numerous to mention. Broadly speaking, however, improved techniques in this art have been based largely on the well-known and fundamental Eccles-Jordan trigger circuit using an inherently bistable pair of cross-coupled triodes. To meet specific requirements involving such factors as rapid counting rates, high stability and precision timing, numerous extensions of the basic circuit have been conceived using pairs of pentodes, additional coupling tubes and the like. But with little exception, all variants of the original have resulted in only more complex circuit arrangements which, though advantageous in one or more respects, effectively reduce over-all reliability due to the increased likelihood of component failure.

There have been prior attempts at reducing the number of tubes, and more particularly, the number of cathodes, necessary to achieve bistable trigger circuit operation. The recent development of bistable magnetic cores has made it possible to use a single triode in circuit with at least one such core to attain a bistable condition. Among the disadvantages which have been encountered in connection With this development are slow speed operation, due to the inherently lengthy switching time for a magnetic core, the comparatively high cost of toroidal core winding, and the lack of a satisfactory indication of the state of the core.

Even prior to the availability of static bistable components, such as magnetic cores, circuits were known which could provide a bistable efiect with a single tube. Thus, for background information, reference is made to a paper by Herbert J. Reich appearing in The Review of Scientific Instruments, volume IX, 1938, page 223, Fig. 2. Here, Reich discloses a single tube pentode trigger circuit which offers two relatively stable values of plate current. It should be observed, however, that this circuit requires a multiplicity of independent power supplies, and further, is not capable of stable operation at the relatively high frequencies demanded of modern counting equipment.

The present invention contemplates and has as a pri mary object the provision of a bistable circuit embodying a single pentode and oflfering two distinctive and highly stable current states for application as an economical binary counter or trigger tube applicable as a substitute for the more customary twin tube circuit techniques. To insure dependable and unambiguous operation, one of the stable states is clearly defined by pentode plate current cut-off while the other stable state is evidenced by substantially full plate conduction. In this respect, the outward effect resembles the more common twin tube trigger circuits.

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Another object of the present invention is to provide a novel pentode bistable circuit which may be switched at exceptionally high frequencies by the application of suitable trigger pulses. It will be seen that individual stages utilizing the concepts of the present invention may be readily cascaded to provide a counting chain of any desired length with a minimum number of tubes and auxiliary circuit components.

Another object of the present invention is to provide a single tube trigger circuit capable of operation from a single direct current power source whose relative amplitude may be varied over an extreme range without adversely affecting performance specifications.

These and other objects of the present invention will become apparent from the following detailed specification when taken in connection with the accompanying drawing in which:

Fig. l is a schematic circuit diagram of the novel bistable circuit of this invention;

Fig. 2 is a graphical representation of the current characteristics of certain electrodes of the electron tube used in Fig. 1; and

Fig. 3 is a graphical representation of potential waveforms at selected points of the circuit shown in Fig. 1.

With reference now to the drawing, and more particularly to Fig. 1 thereof, there is illustrated a circuit which offers two readily distinguishable stable conditions of plate current. Pentode 11 is the sole electron tube used in this circuit and, as is conventional, comprises a cathode, plate, and control, screen and suppressor grids.

Load resistor 12 couples the plate of tube 11 to the positive power source 5+, and the cathode is grounded through unbypassed resistor 13. A voltage divider made up of a series chain of four resistors 14, 15, 16 and 1'7 between 3+ and ground furnishes operating potentials for various components. Thus, the screen grid is connected to the junction of resistors 14 and 15 while the suppresspr is connected to the junction between resistors 15 and 15. Capacitor 21 shunts resistor 15 for reasons which will be discussed below.

A second resistive voltage divider comprising resistors 22, 23 and 24 is also used to establish needed operating potentials. connected to the junction of resistors 23 and 24 and is further couple-d by diode rectifier 25 to the junction between resistors 16 and 17. Diode 25 is preferably a germanium crystal and conventional notation is used to indicate that it is poled to conduct when the junction between resistors 16 and 17 is more positive than the c-pposite junction between resistors 23 and 24.

input signals are applied at terminal 26 to a differentiating circuit made up of capacitor 27 and resistor 28. Diode 31, connected between the difierentiating circuit and the junction between resistors 22 and 23, is arranged for transfer of. negative pulses only through coupling capacitor 32 to the control grid of pentode 11. The output of the bistable stage shown is derived at terminal 33 connected to the tube plate.

Having described the physical configuration of the circuit shown in Fig. 1, the nature of the pentode tube itself will be considered before entering a full discussion of the principles of operation.

In Fig. 2, typical plate and screen current characteristics for pentode 11 are depicted as a function of the potential of the suppressor grid. For the characteristic curves shown, all potentials other than the suppressor are held constant. With a pentode such as type 6AS6, a substantially constant cathode current is observed during variations of suppressor potential. The effect of the latter variation, however, is to alter the division of cathode current between plate and screen. With reference to Fig. 2,

The control grid of pentode 11 is directly it may be seen that with the suppressor quite negative (condition A), plate current cut-off is achieved with maximum screen current. As the suppressor is made more positive, plate current rises with a consequent fall in screen current until condition B is reached. Here, screen current is a reasonably constant minimum while the plate current has risen to a stable maximum.

In accordance with the concepts of the present invention, advantage is taken of this current division phenomenon; and by appropriate choice of components as will be discussed, the pentode plate current is alternately switched between substantially full conduction and cutoff by the application of successive triggers of like polarity.

Consider a rectangular wave, such as illustrated in Fig. 3(A), applied as an input at terminal 26. This waveform is differentiated and by virtue of diode 31. will a pear as a succession of negative triggers, Fig. 3(B), at the pentode control grid.

Assume that initially the status of pentode 11 is that indicated in A in Fig. 2 with heavy screen conduction and plate current cut-off. Under these conditions, output terminal 33 is at the potential of the power source 3+. The application of a negative trigger to the control grid of pentode 11 will switch the plate and screen currents from those of condition A to B, Fig. 2. With the full plate current, the potential of terminal 33 will fall to a minimum value. The next successive negative trigger applied to the control grid of pentode 11 will instantaneously return the tube to condition A, Fig. 2, giving rise to an output square wave, Fig. 3(C), at terminal 33 of half the frequency of the input wave.

, To simplify the explanation of the underlying principles governing operation of the pentode bistable circuit shown in Fig. 1, typical values of circuit potentials appearing in a practical embodiment of this device will be considered. As an aid to following those values during the subsequent discussion, all key potentials have been placed upon Fig. 1 of the drawing. Thus, it will be observed that B+ has been set typically at 350 volts and, due to the constant current effect noted earlier, the cathode potential remains relatively fixed at 92 volts. The control grid is set at 90 volts by the aforementioned voltage divider.

The plate, screen, suppressor and the junction of resistors 16 and 17 and crystal diode 25 each have two stable potential values corresponding to the two stable states available. The starred number at each'of these points indicates the potential during plate current conduction and the unstarred value, the potential during plate current cut-off.

Examining these voltages further, it may be observed that with full plate conduction, the plate potential is maintained at 200 volts while the suppressor is at the more positive condition of 97 volts. Since, as demonstrated by condition B, Fig. 2, most of the cathode current flows in the plate circuit, the screen with its relatively low current is at its higher potential figure or 230 volts. The low screen current is further reflected as a relatively high potential of 85 volts at the junction between resistors 16 and 17.

For the static conditions in force during plate conduction, crystal diode 25 is rendered non-conducting since it is negatively biased by the potentials shown. However, since the bias is only volts, a negative trigger in excess of this amount applied at the control grid will cause conduction in diode 25 which, through resistor 16, will tend to lower the suppressor potential. The latter effect will tend to increase screen current, which, in turn, will lower the screen potential together with the potential at the junction between resistors 15 and 16. The ultimate effect is regenerative with capacitor 21 speeding the transfer of switching signals, and as a result, the potentials of the plate, screen, suppressor and the potential at the junction between resistors 16 and 17 will abruptly switch to those unstarred values indicated on the drawing, to effect a reversal in stable state.

In this stable state, diode 25 is negatively biased by a potential of 28 volts. The next negative trigger applied to the control grid will not overcome this bias, but its effect is a tendency to reduce cathode current and hence, screen current. The potential of the screen thus rises, and, through capacitor 21, transfers a positive leading edge to the suppressor. This effect now regenerates to cause maximum plate conduction with a minimum of screen current, that is, condition B, Fig. 2. In other Words, consecutive negative triggers at the control grid will switch the circuit shown between stable conditions A and B.

Evidently, any number of stages such as shown in Fig. 1 may be cascaded to form a multi-stage binary counter. Thus. if the rectangular wave output appearing at terminal 33 were applied to the equivalent of terminal 26 in a successive stage, the rectangular output waveform of the first stage would be differentiated and the negative triggers thereby generated would switch stable states of the pentode in the next stage. Each stage will effectively divide the frequency of the preceding stage by two. As a switch circuit, plate load resistor 12 may be replaced by a relay, the desirable characteristic being the effective constant current source provided by the pentode during periods of plate conduction. Of particular interest, the relative capacitance of the plate load does not affect switching speed as in conventional circuits because the potential of the plate of a pentode has a minimum effect on the plate current characteristics thereof.

For completeness, there is tabulated the values of circuit parameters which, when used in the circuit of Fig. 1 with B+ at 350 volts, provide the potentials earlier noted in this discussion:

As noted above, pentode 11 is preferably a type 6AS6, but may be one of various other pentodes exhibiting definite suppressor grid control characteristics, such as the types 7AK7 and 6L7. Note that for the potentials indicated on the drawing, a 150 volt output swing appears.

at the plate output terminal 33. Were the output derived at the screen instead, an 80 volt step would be available- This high level of output is highly advantageous in numerous trigger circuit applications.

Of particular interest is the fact that through use of the circuit shown, the need for regulated power supplies is wholly avoided. For the circuit parameters tabulated above, in use with a 6AS6 pentode, excellent performance was available throughout the extreme limits of B+ from 80 to 400 volts. All that was required Was a trigger amplitude of at least 2 volts for the minimum B-land 15 volts for the maximum B+ indicated.

If a source of sharp negative triggers is directly available, then the differentiating circuit 27-28 and diode 31 may be omitted and the triggers applied directly to the control grid of the pentode.

Since numerous modifications and departures may now be made by those skilled in this electrical art, the invention herein is to be construed as limited only by the spirit and scope of the appended claims.

What is claimed is:

1. A bistable electronic circuit comprising, a pentode electron tube having a cathode, plate, and control, screen and suppressor grids, means for applying unipolar trigger signals to said control grid for switching stable states, means including a capacitor coupling said screen and suppressor grid, and means including a normally non-conducting unilateral circuit element for coupling said control grid to said suppressor grid to apply alternate trigger signals to the latter.

2. A bistable electronic circuit comprising, a pentode electron tube having a cathode, plate, and control, screen and suppressor grids, means for applying potentials to said tube electrodes for establishing a cathode current and for diverting said entire cathode current substantially to said screen grid with plate current cut-off, a resistancecapacitance'circuit coupling said screen and suppressor grids, and a rectifier element coupling said control grid to said suppressor grid, said circuit being arranged whereby upon the application of a negative trigger to said control grid, a relatively large portion of said cathode current is abruptly switched to said plate, said circuit being further arranged whereby upon the application of the next consecutive negative trigger to said control grid, said rectifier element will couple said control grid to said suppressor grid for restoring the initial condition of plate current cut-oft with substantial screen grid current.

3. A bistable electronic circuit comprising, a pentode electron tube having a cathode, plate, and control, screen and suppressor grids, a positive potential source, a reference potential point, a load resistor coupling said plate to said source, a cathode resistor coupling said cathode to said reference point, a voltage divider extending between said source and said reference point, means for connecting said suppressor grid to a point on said voltage divider, means for connecting said screen grid to a point on said voltage divider more positive than said suppressor grid, :1 rectifier element coupling said control grid to said suppressor grid, means for applying unipolar trigger pulses to said control grid for actuating said bistable circuit, means including said voltage divider for normally biasing off said rectifier whereby alternate trigger pulses applied to said control grid are coupled through said rectifier element to said suppressor grid. I

4. A bistable electronic circuit comprising, a pentode electron tube having a cathode, plate, and control, screen and suppressor grids, a positive potential source, a reference potential point, a load resistor coupling said plate to said source, a cathode resistor coupling said cathode to said reference point, a voltage divider formed of first, second, third and fourth resistors serially connected between said source and said reference point, said screen grid being connected to the junction between said first and second resistors, said suppressor grid being connected between the junction of said second and third resistors, a rectifier element connected between, and poled for the transmission of negative signals from said control grid to the junction between said third and fourth resistors, means for applying a static potential positive relative to said reference point to said control grid, means for applying negative trigger pulses to said control grid for actuating said bistable circuit, said voltage divider being arranged to provide a potential at said junction between said third and fourth resistors for normally biasing off said rectifier element, whereby alternate negative trigger pulses applied as aforesaid to said control grid are coupled through said rectifier element and said third resistor to said suppressor grid.

5. A bistable electronic circuit comprising, a pentode electron tube having a cathode, plate, and control, screen and suppressor grids, a positive potential source, a reference potential point, a load resistor coupling said plate to said source, a cathode resistor coupling said cathode to said reference point, a voltage divider formed of first, second, third and fourthresistors serially connected between said source and said reference point, said screen grid being connected to the junction between said first and second resistors, said suppressor grid being connected between the junction of said second and third resistors, a rectifier element connected between, and poled for the transmission of negative signals from said control grid to the junction between said third and fourth resistors, means for applying a static potential positive relative to said reference point to said control grid for normally biasing off said rectifier element, means for applying negative trigger pulses to said control grid for actuating said bistable circuit, the values of said resistors relative to the potential difference between said source and said reference point being arranged whereby alternate negative trigger pulses applied as aforesaid to said control grid are coupled through said rectifier element and said third resistor to said suppressor grid.

6. Apparatus as in claim 5 and including a capacitor shunting said second resistor.

7. A pentode circuit capable of assuming first and second stable states in response to successive unipolar triggers applied to the control grid'thereof, comprising means coupling the screen and suppressor grids of said pentode to produce a division of the tube current flow, means for applying potentials to the tube electrodes to control the extent of said division of tube current flow, and a diode element intercoupling the control and suppressor grids of said tube and biased to conduct solely during the application to said circuit of alternate triggers.

References Cited in the file of this patent UNITED STATES PATENTS 2,060,095 Mathes Nov. 10, 1936 2,143,397 White Ian. 10, 1939 2,275,016 Koch Mar. 3, 1942 2,404,918 Overbeck July 30, 1946 2,404,919 Overbeck July 30, 1946 2,549,874 Williams Apr. 24, 1951 2,584,882 Johnson Feb. 5, 1952 2,692,334 Blumlein Oct. 19, 1954 FOREIGN PATENTS 576,469 Great Britain Apr. 5, 1946 588,417 Great Britain May 21, 1957 

